Primary central nervous system lymphoma (PCNSL) is an extranodal variant of non-Hodgkin lymphoma (NHL) that is confined to the nervous system. The diagnostic and therapeutic approaches to PCNSL are different from those that are applied to other forms of NHL or to other brain cancers. The International Primary Central Nervous System Lymphoma Collaborative Group (IPCG) consists of specialists from multiple countries and disciplines dedicated to the study of PCNSL. The IPCG holds an annual scientific conference at the American Society of Hematology meeting in which various research topics related to PCNSL are discussed. An international expert on the topic under discussion is selected each year by an executive committee to chair the scientific conference. These meetings have led to multiple collaborative projects that have positively impacted the field and that have been published in the peer- reviewed literature. The annual IPCG meeting is a critical forum for generating new ideas and approaches to this unique form of brain cancer and lymphoma. The 2015 conference topic is Evaluation and Response Criteria for Relapsed and Refractory Primary CNS Lymphoma, an understudied and critical topic as most patients with PCNSL are refractory to treatment or subsequently relapse after initial therapy.